In any of various image forming apparatuses using an electrophotographic method, such as a laser printer, electrostatic copier, plain-paper facsimile apparatus, or a multi-function printer combining the above, a developing roller is used. The developing roller develops an electrostatic latent image, formed on a surface of a photoreceptor, into a toner image by means of a toner.
As the developing roller, it is effective to use a semiconductive roller, with which the roller resistance value is adjusted to not more than 108Ω. This is because the toner can thereby be imparted with a high charging property in correspondence to trends toward fine, uniform, and spherical toners and transition to polymerized toners and the electrostatic latent image can be developed efficiently into the toner image without causing attachment of toner.
In order to answer the various needs regarding the semiconductive roller, various types of rubber component that forms the semiconductive roller, types and blending proportion of additives, structures, etc., are being examined.
For example, to manufacture a semiconductive roller at as high a productivity and as low a cost as possible and improve its durability, compression set, etc., the semiconductive roller is preferably formed to have a non-porous single layer structure. If this structure is to be adopted, it is preferable to use an ion conductive rubber, such as epichlorohydrin rubber, etc., as the rubber component so suppress decrease of the toner charge amount and thereby obtain an image of high an image quality as possible.
For example, in Patent Document 1 (Japanese Patent Publication No. 4688936), the two types of rubber components of epichlorohydrin rubber and chloroprene rubber (CR) are used in combination to form a semiconductive roller. However, when the semiconductive roller that uses just these two types of rubber components in combination is used as the developing roller, decrease of the image density of the formed image occurs due to attachment of toner.
In Patent Document 2 (Japanese Patent Application Publication No. 2007-72445), it is proposed that a semiconductive roller contain a filler (titanium oxide, etc.) having a function of preventing the attachment of toner to suppress the decrease of the image density due to attachment of toner and thereby secure an appropriate image density. However, when the filler is blended at an amount at which the abovementioned function is obtained sufficiently, the hardness of the semiconductive roller increases to cause a new, different problem. That is, degradation of toner occurs readily to cause decrease of image durability, and the nip width when the semiconductive roller is press-contacted with a surface of a photoreceptor becomes narrow and the image quality of the formed image may thereby be degraded. The image durability is an index expressing how long the image quality of the formed image can be maintained satisfactorily when the same toner is repeatedly used to perform image forming.
In one time of image forming, just a portion of the toner housed in a developing portion of an image forming apparatus is used and the remaining large portion of toner is repeatedly circulated inside the developing portion. Therefore a major key to improving the image durability is how much damage is applied or not applied to the toner by the developing roller that is disposed inside the developing portion and repeatedly contacts the toner.
When the image durability decreases, a white vertical stripe forms in a solidly black portion of the formed image or fogging occurs at a margin portion to degrade the image quality.
It may be considered to make the semiconductive roller have a porous structure by blending a foaming agent, etc., and thereby impart the semiconductive roller with high flexibility. However, such a porous semiconductive roller has problems in that it is shorter in durable life than a non-porous roller and must be exchanged due to occurrence of setting, etc., in a comparatively short period of time.
Patent Document 3 (Japanese Patent Application Publication No. 9-114189) discloses a semiconductive roller with a two-layer structure where a surface layer is laminated on an outer peripheral surface of a conductive elastic body layer. The surface layer has a sea-island structure constituted of a mutually incompatible mixture of acrylonitrile butadiene rubber (NBR) and styrene butadiene rubber (SBR) and contains an ion conductivity imparting agent.
Lithium perchlorate, sodium perchlorate, calcium perchlorate, and long-chain alkyl quaternary ammonium perchlorate, etc., are cited as examples of the ion conductivity imparting agent.
Forming of a semiconductive roller with a single-layer structure adopting the arrangement of the surface layer described above may be considered. In this case, an ion conductive rubber is not included and it may therefore be possible to prevent the attachment of toner while maintaining a low roller resistance value by the ion conductivity imparting agent.
However with this arrangement, the ion conductivity imparting agent readily exudes to the surface when, for example, electric field application is continued or upon exposure to high temperature, etc. There is thus a problem in that the exuded ion conductivity imparting agent becomes transferred onto the surface of the photoreceptor, etc., to cause so-called contamination and degrade the image quality of the formed image.
Patent Document 4 (Japanese Patent Application Publication No. 2002-278320) discloses a semiconductive roller with a two-layer structure where a surface layer is laminated on an outer peripheral surface of an elastic layer. The surface layer is made of a fluorine-based material and the elastic layer is made of a mixture of ethylene propylene diene rubber (EPDM), NBR, and SBR and contains conductive carbon black (carbon conductive substance).
However, if electronic conductivity is imparted using just the conductive carbon black as the conductive agent, the roller resistance value cannot be stabilized unless a laminated structure is arranged by coating the outer peripheral surface with the surface layer, etc. That is, the semiconductive roller cannot be arranged as a single-layer structure and there is thus a problem in that the productivity is decreased and the manufacturing cost is made high in correspondence to increase of manufacturing steps and materials used.
Patent Document 5 (Japanese Patent Application Publication No. 2012-163776) discloses a semiconductive roller formed by using SBR and epichlorohydrin rubber in combination as the rubber components.
It is deemed that the effects that                the semiconductive roller can be made pliable to enable improvement of the toner image durability, and        the amount of epichlorohydrin rubber, which is a cause of attachment of toner, can be lessened in correspondence to the combined use of SBR to make it less likely for decrease of image density due to attachment of the toner etc., to occur can be obtained when these two types of rubber components are used in combination.        